LIVERMORE, Calif.—Researchers in Lawrence Livermore National Laboratory’s Atmospheric Science Division have demonstrated a cooling of up to 2-degree Fahrenheit over land between 1000 and 1900 AD as a result of changes from natural vegetation, such as forests, to agriculture.

Through climate model simulations, the LLNL research team made up of Bala Govindasamy, Ken Caldeira and Philip Duffy, determined that a previously recognized cooling trend up to the last century could, in part, be attributed to the land-use change.

Previous studies had attributed cooling to natural climate variations. The Livermore research, however, suggests that much of this cooling could have been the result of human activity.

Forests tend to look dark from the sky, but agricultural lands, with their amber waves of grain, tend to look much lighter. Dark colors tend to absorb sunlight, and light colors tend to reflect sunlight back out to space. Changing from forests to crops results in more sunlight reflected back to space. This reflection of solar energy to space tends to cool the Earth, especially in regions such as the eastern and mid-western United States, where huge tracts of land have been converted to crops. In the 20th century, some of this cropland has been reverting back to forest, especially in the eastern United States.

Greenhouse gas emissions in the 20th century likely overcame any cooling trends that took place up to that time. Growing more trees has been suggested as a way to soak up carbon dioxide, a greenhouse gas, from the atmosphere. However, earlier studies demonstrate that growing dark forests could actually heat the earth’s surface more because dark colors tend to absorb more sunlight, despite the uptake of carbon dioxide.

"The Earth land surface has cooled by about 0.41 K (= by about 3/4 of a degree Fahrenheit) due to the replacement of dark forests by lighter farms growing wheat, corn, etc.," said Caldeira, a climate model researcher who also is co-director for the Department of Energy’s Center for Research on ocean carbon sequestration. "This is an example of inadvertent geoengineering -- we changed the reflectivity of the Earth and have probably caused a global cooling in the past. This is now probably being overwhelmed by our greenhouse gas emissions."

The research, published in the Geophysical Research Letters, also shows a slight increase in the annual means of global and Northern Hemisphere sea ice volumes in association with the cooling. The simulated annual average cooling due to land-use change during this period is almost a half a degree Fahrenheit globally, 0.66 °F for the Northern Hemisphere and .74 °F over land.

In the simulations, land use data for 1000 AD uses potential natural vegetation, made up mainly of forests, while data for the 1900 AD period uses standard current vegetation data, which is a mix of forest and croplands, taken from the Community Climate Model developed at the National Center for Atmospheric Research. The greenhouse gas levels in both simulations are in concentrations taken at pre-industrial levels.

"The estimated temperature change in the continental United States as a result of change from forests to agriculture is up to a 2-degree Fahrenheit cooling," Caldeira said. "So, when we talk about global warming, we can no longer take for granted that this global warming is starting from some natural climate state, undisturbed by human activities."